FIG. 1 is a block diagram illustrating a related art driving system for hybrid electric vehicles.
Referring to FIG. 1, the related art driving system for hybrid electric vehicles includes a battery 10, a DC/DC converter 20, a motor driving unit 30, a power generation unit 40, a PWM controller 50, and a direct current (DC) voltage smoothing capacitor 60.
The battery 10 is a power supply source for supplying a low-level DC voltage.
The DC/DC converter 20 is an element that converts the low-voltage DC power into high-voltage DC power. Since the DC/DC converter 20 converts the low-voltage power into the high-voltage power, the DC/DC converter 20 may be referred to as a high voltage DC/DC converter (HDC).
The motor driving unit 30 includes an inverter that converts the high-voltage DC power, supplied from the DC/DC converter 20, into alternating current (AC) power and a driving motor that is driven with the AC power.
The driving motor included in motor driving unit 30 operates in a motoring mode for driving a vehicle and operates in a regenerative mode for collecting kinetic energy of a vehicle as electric energy.
The power generation unit 40 includes an inverter that converts the high-voltage DC power, supplied from the DC/DC converter 20, into the AC power and a driving motor that is driven with the AC power.
Unlike the driving motor included in the motor driving unit 30, the driving motor included in the power generation unit 40 may mainly operate in the regenerative mode, but may operate in the motoring mode.
The DC voltage smoothing capacitor 60 stabilizes (smooths) the high-voltage DC power supplied from the DC/DC converter 20.
A voltage output from each of the DC/DC converter 20 and the inverters is controlled according to a PWM signal generated by the PWM controller 50.
A duty ratio of the PWM signal is controlled based on a comparison result between a reference signal and a PWM carrier signal. Here, the PWM carrier signal may be a sawtooth-wave signal or a triangle-wave signal.
In the related art driving system for hybrid electric vehicles, PWM carrier signals applied to the DC/DC converter 20 and the inverters have the same phase.
In a method of controlling a phase of a carrier signal, when the driving motor of the motor driving unit 30 and the driving motor of the power generation unit 40 operates in the same operation mode (for example, the motoring mode or the regenerative mode), a ripple component of a current which flows in the DC voltage smoothing capacitor 60 according to an operation of the inverter of the motor driving unit 30 and a ripple component of a current which flows in the DC voltage smoothing capacitor 60 according to an operation of the inverter of the power generation unit 40 is summated, and for this reason, the ripple component of the current flowing in the DC voltage smoothing capacitor 60 increases. The increased rippled component of the current shortens a lifetime of the DC voltage smoothing capacitor 60.
Moreover, a size (or a capacitance) of the DC voltage smoothing capacitor 60 is designed in proportion to the ripple component of the current. Therefore, when the ripple component of the current increases, the size (or the capacitance) of the DC voltage smoothing capacitor 60 increases.
As described above, the reason that the ripple component of the current flowing in the DC voltage smoothing capacitor 60 increases is because the PWM carrier signals applied to the DC/DC converter 20 and the inverters have the same phase irrespective of an operation mode of each of the driving motors.
Therefore, if the PWM carrier signals applied to the DC/DC converter 20 and the inverters are controlled to have different phases according to the operation modes of the driving motors, the ripple component of the current flowing in the DC voltage smoothing capacitor 60 can be reduced.
However, a system for separately controlling the phases of the PWM carrier signals applied to the DC/DC converter 20 and the inverters according to the operation modes of the driving motors is not yet developed.